1. Field of Invention
The present invention relates generally to optical networks. More particularly, the present invention relates to extending the span distances associated with coarse wavelength division multiplexing systems.
2. Description of the Related Art
In response to the ever-growing demand for fiber networks, coarse wave division multiplexing (CWDM) has been developed as an alternative to dense wavelength division multiplexing (DWDM). CWDM systems use uncooled lasers, and therefore allow a greater spacing between wavelengths or channels than DWDM systems. In general, CWDM is a relatively low cost solution that provides connection flexibility and increased throughput for metropolitan networks.
CWDM combines up to sixteen or eighteen wavelengths onto a single fiber, although many CWDM systems combine four or eight wavelengths onto a single fiber. CWDM technology uses an ITU standard approximately 20 nanometer (nm) spacing between wavelengths or channels. The wavelengths are typically between approximately 1270 nm and approximately 1610 nm.
FIG. 1 is a diagrammatic representation of an overall network system that includes a system which communicates using CWDM. A network 120 that is in communication with an overall Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) transport network 104 includes various nodes 124a-d that are in communication over links 128. Network 120 may be a metro network that uses CWDM. Any of nodes 124a-d, as for example node 124c, may be in communication with other nodes 124e, 124f across links 132. Nodes 124c, 124e, 124f may be part of a local area network.
When CWDM is used in network 120, the distances covered by links 128 are typically limited to being less than approximately 100 kilometers (km). That is, CWDM systems are generally limited by an optical span budget of approximately 100 km. To increase the distances over which CWDM communications may occur, optical amplifiers may be incorporated into network 120.
Optical amplifiers such as semiconductor optical amplifiers (SOAs) provide relatively high-speed switching capability and also provide the ability to add gain to optical signals. As will be understood by those skilled in the art, an SOA uses technology that is similar to that of a Fabry-Perot laser diode. In some CWDM systems, SOAs may be used as amplifiers to provide gain to optical signals relatively inexpensively. Adding gain to optical signals enables the optical signals to traverse a greater distance. That is, amplifying optical signals allows the span budget associated with optical signals to be increased.
FIG. 2 is a block diagram representation of a four channel CWDM system that uses an SOA for amplification. Four CWDM channels 204a-d are provided as input into an SOA 208. SOA 208 amplifies channels 204a-d to produce amplified channels 204a′-d′ as an output.
An eight channel CWDM system, on the contrary, may utilize two SOAs which each amplify four channels. Two SOAs may be used due to the limited amplication bandwidth of SOAs. With reference to FIG. 3, a system which utilizes two SOAs in conjunction with a band splitter and a band combiner will be described. CWDM channels 304, 306 are provided to a band splitter 314 which substantially splits channels 304, 306 into low band channels 304 and high band channels 306. Low band channels 304 are then amplified by a first SOA 318, while high band channels 306 are amplified by a second SOA 316. The outputs of SOAs 316, 318 are provided to a band combiner 320 that recombines amplified low band channels 304′ and amplified high band channels 306′.
Although the use of filters such as band splitter 314 and band combiner 320, in conjunction with SOAs 316, 318, is effective in providing amplification for eight channel CWDM systems, the use of the filters introduce additional loss that impacts and reduces the span budget. Filters such as band splitter 314 and band combiner 320 may occupy a significant amount of physical space.
Therefore, what is needed is a method and an apparatus which enables the maximum span budget in a CWDM system to be efficiently increased. That is, what is desired is a system that allows CWDM channels to be efficiently amplified such that the distances which may be traversed by signals traveling on the channels may be increased.